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How to make the menu/nav
resize automatically within a layer to whatever breakpoint is.
I have various breakpoint but I'm not able to fill in/stretch the navigation in-between the breakpoints set. It's often smaller than the layer and on unfold doesn't fill the width of the screen.

Which menu are you using?

WB responsive menu inside a floating layer/header

If you use the menu inside a layout grid and set it to 'full width', then it will automatically adapt to the size.